This invention relates to a process for gasifying coal using steam, lime, and stabilized Group I metal salt catalysts. More specifically, the invention also concerns stabilized Group I metal salt catalysts using fluoride salts of Group II metals such as CaF.sub.2 for catalyst stabilization.
Early work involving the production of hydrogen from coal has been disclosed in British Patent 8743 (1910). This patent showed a reaction with coal char and steam to produce hydrogen and CO.sub.2. The reaction mixture contained lime as a carbon oxide acceptor and an alkali salt catalyst such as K.sub.2 CO.sub.3, borax, Na.sub.2 CO.sub.3, etc. For convenience, this process is termed the catalytic-steam-carbon-lime process (CSCL).
Other techniques for producing hydrogen are known and include the steam-oxygen processes which basically produce a synthesis gas composed mainly of carbon monoxide and hydrogen. This is achieved by a high temperature endothermic reaction of steam with carbon (H.sub.2 O + C .fwdarw. CO + H.sub.2) and the exothermic reaction of oxygen with carbon (C + 1/2 O.sub.2 .fwdarw. CO). The synthesis gas produced must be catalytically shifted to hydrogen and CO.sub.2 through the lower temperature water gas shift reaction (CO + H.sub.2 O .fwdarw. CO.sub.2 + H.sub.2). Carbon dioxide and sulfur gases are then removed from the gas stream by liquid phase absorption to yield relatively pure hydrogen gas. Lurgi, Koppers-Totzek and Winkler type gasifiers could all be used for these processes to produce hydrogen.
One major disadvantage of the steam-oxygen processes is that the steam-oxygen gasifiers operate at very high temperature (3500.degree. F in the case of Koppers-Totzek) in comparison to the CSCL process which employs a reaction temperature of about 1200.degree. F - 1400.degree. F.
Furthermore, oxygen is required in the gasifiers to provide the energy required for the endothermic steam-carbon reaction. In the case of Koppers-Totzek, approximately 0.8 tons of oxygen is required for every ton of coal gasified. By comparison, no oxygen is required in the CSCL process.
Finally, separate water-gas, shift reactor and CO.sub.2 and sulfur gas removal is required for the steam-oxygen processes. By comparison, sulfur and CO.sub.2 removal takes place "in-situ" in the CSCL process.
Another technique for producing hydrogen is by the steam-iron process which produces hydrogen through the reaction of steam with iron (3Fe + 4H.sub.2 O .fwdarw. Fe.sub.3 O.sub.4 + 4H.sub.2). The iron is then regenerated by reduction of the iron oxide product with a clean synthesis gas (CO + H.sub.2). The primary disadvantage of this scheme in comparison with the CSCL process is that 1/2 - 3/4 of a mole of clean (sulfur free) impure hydrogen (CO + H.sub.2) is required to regenerate the iron oxide for every mole of pure hydrogen which is produced. On the other hand low grade, high ash residual char can be used to regenerate the lime in the CO.sub.2 acceptor scheme.
A basic problem, however, exists with the CSCL process. This problem involves the deactivation of the Group I metal salt catalyst which tends to be poisoned by reaction at high temperature with silica, alumina or alumino-silicates (in the coal ash) in the gasifier or in the lime regenerator. British Patent 8743 (1910) referred to, supra, did not teach how to stabilize the catalyst to prevent deactivation or how to regenerate the lime.
It is, therefore, an object of this invention to provide a CSCL process and apparatus for producing hydrogen in which the lime is regenerated and the catalyst remains active and, hence, reusable.
Another object is to provide a CSCL process and apparatus for producing hydrogen rapidlyat 1200.degree. F - 1400.degree. F via the reaction: ##STR1##
Another object is to provide a CSCL process and apparatus having high reaction selectivity to hydrogen, with carbon dioxide being the only other significant reaction product. A sufficiently low reaction temperature would permit most of the carbon monoxide initially formed to be shifted through a reaction with steam (CO + H.sub.2 O .fwdarw. CO.sub.2 + H.sub.2) to carbon dioxide and sorbed by the lime.
Another object is to provide a CSCL process and apparatus wherein the hydrogen production reaction is slightly exothermic and, hence, no requirement is necessary for heat addition (or oxygen addition) in the gasifier.
Another object is to provide a CSCL process and apparatus having a lime regeneration reaction ##STR2## which can be completely decoupled from the hydrogen production reaction. This enables use of a variety of low cost energy sources (e.g., combustion of the residual high ash content char with air rather than oxygen) to effect the limestone decomposition.
Another object is to provide a CSCL process having a reactive system in which sulfur compounds which are present in coal are retained in the lime/limestone mixtures thereby eliminating the necessity for a separate sulfur removal system.
Another object is to provide a CSCL process and apparatus having a reaction system which is capable of recycling stabilized catalysts many times between a primary reactor and a lime regenerator without losing their activity.
Another object is to provide new and improved catalysts which may be employed in the CSCL process wherein the catalysts are able to: 1) gasify less reactive chars in addition to the highly reactive lignites that are used primarily in the present systems; and 2) operate at lower temperatures thereby resulting in greater retention of both carbon oxides and sulfur compounds in the lime without coking or fusion problems.